1. Field of the Invention
This invention relates to a method for the production of a hydrophilic resin and to a novel absorbent resin.
More particularly, this invention relates to a method for the production of a hydrophilic resin characterized by the steps of copolymerizing a hydrophilic unsaturated monomer with a cross-linking agent possessing a specific structural unit and subjecting the resultant crosslinked copolymer to a heat treatment and to an absorbent resin capable of changing the absorption capacity thereof upon being heated.
The method of production according to this invention offers a convenient solution to the various problems encountered by the conventional method of production, simplifies the process of production, and not only realizes a prominent improvement in productivity but also enables the produced hydrophilic resin to have various improved properties.
Further, the absorbent resin of this invention, though dry in constitution, is endowed with the heretofore unattainable characteristic of having the absorption capacity thereof changed upon being heated. Thus, it promises utility in a wide range of novel applications. The absorbent resin of this type, after being used to capacity, can be easily solubilized and decomposed and therefore discarded easily and safely.
2. Description of the Prior Art
Generally, the hydrophilic resins are broadly classified by solubility under water-soluble resins and absorbent resins.
The water-soluble resins which have been heretofore known to the art include polysodium acrylate (JP-B-48-42,466 and JP-B-42-9,656), polyacrylic acid and polyacryl amide (JP-A-54-145,782 and JP-A-57-18,652), polymer of 2-acryl amide-2-methyl propane sulfonic acid ( JP-A-2-173,108), partially hydrolyzed polyacryl amide (JP-A-52-137,483), acrylic acid-acryl amide copolymer (JP-A-59-15,417) , (meth)acrylic acid-itaconic acid copolymer (JP-A-58-91,709), and polyvinyl alcohol, for example.
The absorbent resins are water-insoluble hydrophilic resins which are gelled by absorbing water and have found extensive utility in the field of hygienic articles such as disposable diaper and sanitary napkins, in the field of agriculture and forestry, and in the field of civil engineering, for example.
The absorbent resins of this type which have been known heretofore to the art include partially neutralized cross-linked polyacrylic acids (JP-A-55-84,304, JP-A-55-108,407, JP-A-55-133,413, JP-A-56-84,632, and JP-A-57-34,101), hydrolyzed starch-acrylonitrile graft polymer (JP-A-46-43,995), neutralized starch-acrylic acid graft polymer (JP-A-51-125,468), saponified vinyl acetate-acrylic ester copolymer (JP-A-52-14,689), hydrolyzed acrylonitrile copolymer or acryl amide copolymer (JP-A-53-15,959) or cross-linked compounds thereof, cross-linked cationic monomers (JP-A-58-154,709 and JP-A-58-154,710), and crosslinked copolymer of 2-acryl amide-2-methyl propane sulfonic acid with acrylic acid (JP-A-58-2,312), for example.
In the methods used for producing these hydrophilic resins, the procedure which comprises preparing the solution of a hydrophilic unsaturated monomer in a solvent such as water, subjecting the solution to aqueous solution polymerization or reversed-phase suspension polymerization thereby obtaining a gel-like polymer, and optionally drying the polymer is generally employed. As a natural consequence, the production of such a hydrophilic resin is required to handle a gel-like polymer. Since the gel-like polymer which is not cross-linked at all or is cross-linked only sparingly is extremely soft and viscous, however, it poses numerous problems to various steps of the process of production such as polymerization, pulverization, conveyance, and drying.
As a solution of these problems, it is possible to consider adopting a method for enabling the hydrophilic resin to acquire a greatly increased cross-linked density. This method is capable of solving the problems mentioned above because it causes the gel-like polymer to acquire increased gel strength and consequently a lowered viscosity. The incorporation of a high degree of cross-linking to the water-soluble resin has no relevance in the first place. Further, the useless increase of cross-linked density in the absorbent resin only brings about a great decrease in the resin's absorption capacity and keeps the resin from acquiring the desired high absorption capacity. In many cases, the conspicuous decrease of the absorption capacity renders the absorbent resin no longer capable of withstanding the impact of actual use.
Further, it has been heretofore customary for the absorption capacity of a given absorbent resin to be adjusted exclusively by altering the conditions of production such as the amount of a cross-linking agent used in the production of the polymer. It has been impossible to impart an increased absorption capacity to the absorbent resin in the dry state in which the resin is obtained as a finished product. A manufacturing, process for obtaining an absorbent resin having a specific absorption capacity, therefore, must control complicated and delicate conditions of production. Generally, consumers of absorbent resins, such as diaper producers, are not in a position to effect a desired adjustment in the absorption capacity of an absorbent resin and therefore, have no alternative but to procure absorbent resins having widely varying absorption capacity fit for numerous uses.
The technique of cross-linking the surface region of an absorbent resin has hitherto been known to the art in order to improve the absorption properties of an absorbent resin on ideally balanced levels contrary to these absorbent resins. Further, a number of methods have been proposed to date for the realization of this improvement.
For example, methods which use a polyhydric alcohol (JP-A-58-180,233 and JP-A-61-16,903), a method which uses a polyglycidyl compound, polyaziridine compound, polyamine compound, or polyisocyanate compound (JP-A-59-189,103), a method which uses a glyoxal (JP-A-52-117,393), methods which use polyvalent metals (JP-A-51-136,588, JP-A-61-257,235, and JP-A-62-7,745), methods which use silane coupling agents (JP-A-61-211,305, JP-A-61-252,212, and JP-A-61-264,006), a method which uses an epoxy compound and a hydroxy compound (JP-A-2-132,103), and a method which uses an alkylene carbonate (DE-4020780) separately as a cross-linking agent have been known to the art. Besides, methods which resort to the presence of an inert inorganic powder (JP-A-60-163,956 and JP-A-60-255,814), a method which resorts to the presence of a dihydric alcohol (JP-A-1-292,004), and a method which resorts to the presence of water and an ether compound (JP-A-2-153,903) separately during the process of the cross-linking reaction have been also known to the art.
Though these methods improve the balance of various properties of an absorbent resin to a certain extent, the improvement obtained hardly deserves to be called satisfactory. The improved absorbent resins which are produced by these methods have room for further enhancement in quality. Particularly in recent years, the need to develop a resin which maintains at a high level, the absorption capacity without load, one of the fundamental physical properties inherent in the conventional absorbent resin, and also excels in the absorption properties under load, particularly the absorption capacity under load, has come to find widespread recognition. As a matter of course, the absorption capacity without load and the absorption capacity under load generally contradict each other. The heretofore known conventionally techniques for the surface cross-linking of an absorbent resin are such that they cannot fully satisfy this need.
In addition to the problems regarding the production and properties on use of a hydrophilic resin, the problem of safe disposal of a used hydrophilic resin has attracted increasing interest as a public issue.
In the discarding of disposable diapers which are frequently used in huge quantities, for example, the practice of burying such used disposable diapers in soil is now popular. The gel of an absorbent resin which, unlike pulp, has a cross-linked structure persists a long time in an undecomposed state in soil. Thus, techniques for liquefying the absorbent resin by irradiating the gel of the absorbent resin with an ultraviolet light (JP-A-1-231,983) and adding an oxidizing agent to the gel (JP-A-1-284,507), for example, have been proposed. These methods, however, entail a complicated procedure and, moreover, fail to allow sufficient decomposition of the absorbent resin.
The present invention is accomplished in order to prove such situation, so an object of this invention is to provide a convenient method for the production of a hydrophilic resin excellent in various properties, which method allows the individual steps of the process of production to proceed stably at all times irrespective of the kind of hydrophilic resin.
Another object of this invention is to provide a novel absorbent resin which, even in the form of a finished product, enables the absorption capacity thereof to be elevated at will.
Yet another object of this invention is to provide an absorbent resin which exhibits as high an absorption capacity under load as in the absence of load and, particularly when it is used as a material for sanitary articles, exhibits ideal properties and a method for the production of the absorbent resin.
Still another object of the present invention is to provide an absorbent resin capable of being decomposed when it is wasted.